The Force is the key mystical power in the Star Wars universe. Used by Jedi, Sith and other groups, it encompasses all living beings and gives special abilities to those who master it. And mastering it is not an easy task. Though some folks are naturally gifted with Force-sensitivity, even those with the highest midi-chlorian count must practice and study it. In Star Wars Legends, when the Force was initially discovered, it was quite difficult to learn being that no one had ever practiced it before. In order to learn its ways, scholars broke it down into The Four Aspects of the Force: the Living Force, the Unifying Force, the Cosmic Force and the Physical Force. These Four Aspects not only allowed a student to better grasp this elaborate concept, it also influenced the way a person would use the Force, depending on if they ascribed to one aspect over the other.
Here are the Four Aspects of the Force explained through Legends (and a brief part from canon).
The Living Force
The Living Force encouraged its users to let go of emotions and embrace a "live in the moment" attitude. The Living Force represents the idea that the Force is here and now, and the present must be embraced. Too much focus on the future creates turmoil. Encompassing all life forms, the Living Force also represents the tangible Force that can be felt in other individuals. Qui-Gon Jinn was the biggest proponent of this practice.
The Unifying Force
The Unifying Force unifies the universe as a whole, somewhat similar to the Living Force, but in actuality the practice is quite different. Those who practice the Unifying Force believe that there is neither a Light Side nor Dark Side. Rather, the Force treats everyone as equal and nothing is all good or all bad.
In stark contrast to the ways of the Living Force, which embraces the idea of the present, followers of the Unifying Force look towards the future. They focus on destiny rather than the here and now. Therefore, visions are commonplace for those who practice it. Yoda was the most prominent follower of the Unifying Force.
The Cosmic Force
Becoming one with the Force is a common goal among its users in Star Wars. This is where the practice of the Cosmic Force is most helpful. The Cosmic Force is the energy that connects a being to the Force. Through it, a Jedi can learn to become a Force ghost.
In Season 6 of Star Wars: The Clone Wars, Qui-Gon explains to Yoda that it is through this practice that he can appear to the Jedi Master after death. "I am a manifestation of the Force, a Force that consists of two parts. Living beings generate the Living Force, which in turn powers the wellspring that is the Cosmic Force." He continues, "All energy from the Living Force from all things that have ever lived, feeds into the Cosmic Force, binding everything and communicating to us through the midi-chlorians. Because of this I can speak to you now."
The Physical Force
Force push. Force lightning. Force dyad. Force telepathy. These are just a few of the many Force abilities utilized in Star Wars. They are also examples of the Physical Force. The Physical Force is what allows one to use the power in a physical manner.
While all of the Four Aspects of the Force are different, they complement one another. Obi-Wan put it best in A New Hope when he tells Luke, "It’s an energy field created by all living things. It surrounds us and penetrates us; it binds the galaxy together.”